The invention relates generally to the production of hollow bodies and more especially preforms for forming hollow bodies, by extrusion blow molding.
An apparatus for the production of hollow bodies from thermoplastic material, the wall of which comprises a laminate having at least first and second layers, by means of extrusion blow molding, may typically comprise an extrusion unit including at least one extruder and an extrusion head having a housing which contains a storage chamber for the plasticised material and has an annular discharge or extrusion opening leading from the storage chamber, for extrusion of a preform. To give a more detailed picture of an apparatus of that nature, reference may be made to DE-A-16 29 104 in which moreover the piston is provided in its peripheral surface with a groove-like recess extending parallel to the direction of movement of the piston. With the housing, the recess forms at least one receiving passage which is disposed opposite at least one opening in the housing of the extrusion head, for the intake of thermoplastic material into the extrusion head from the at least one extruder. The length of the groove-like recess approximately corresponds to the length of the stroke movement of the piston. As the design configuration of that apparatus is such that the passage for receiving the material from the extruder is disposed opposite to the inlet opening in the housing of the extrusion head, the plastic material which passes into the housing through the inlet opening firstly flows into the receiving passage in which it then flows towards an adjoining delivery duct and into the storage chamber.
However that apparatus suffers from the disadvantage that the plasticised material coming from the extruder flows through the receiving passage over the entire length thereof, only in one position of the piston, namely the position in which the piston is in its limit position of being towards the discharge opening of the extrusion head. In all other positions of the piston, a portion of the receiving passage which is delimited on its outside by the housing is disposed at the side of the inlet passage at which it cannot communicate with the above-mentioned delivery passage, with the result that the material which enters through the inlet opening in the extrusion head cannot flow through the above-mentioned portion of the receiving passage, which is taken out of operation by virtue of the positioning of the piston. It will be appreciated that the extent of that portion of the receiving passage varies with the actual position of the piston. That consideration is particularly disadvantageous when, due to the operating procedure involved, the piston performs a stroke movement in which the material entering through the inlet opening of the assembly does not pass through the receiving passage over the entire length thereof, in any end position of the movement of the piston. However, even under other conditions of operation, it is inevitable that, in each operating cycle, the receiving passage contains material which suffers stagnation, for a given period of time; it should be noted in this respect that the material which, at the beginning of the filling stroke movement performed by the piston, first passes into the portion of the receiving passage through which plasticised material no longer flows, is last to be displaced out of the receiving passage, in the following emptying stroke movement of the piston, by the fresh material which subsequently flows into the extrusion head through the inlet opening. This `first-in/last-out` situation for the material in the receiving passage can easily result in the material being processed suffering from a deterioration, even if the residence times of the stagnating material in the head are very short, particularly when the plastic materials involved are sensitive. Such a deterioration of the material used can in turn result in marked deteriorations in the quality of the finished products made from the preforms produced by means of the adversely affected material.
Similar considerations also apply in regard to another form of apparatus for producing hollow bodies from thermoplastic material, as disclosed in German published specification (DE-AS) No 21 61 356, in which the thermoplastic material is introduced into the extrusion head in the same fashion. The only difference in this apparatus, in comparison with that described above, is that the piston is of an annular configuration in cross-section, thus providing outer and inner peripheral surfaces.
Still another form of apparatus for producing hollow bodies from thermoplastic material, as disclosed in German published specification (DE-As) No 26 25 786, seeks to avoid the disadvantages of the two apparatuses referred to above. Therein the feed of thermoplastic material occurs axially into the piston which is also of an annular configuration, using telescopic tube members, the length of which is so selected that they are capable of bridging over and thus compensating for the stroke movements of the piston. However that arrangement suffers from the disadvantage that the relative movement as between the piston and the telescopic tube members means that the latter also produce a piston effect, and that can have a disadvantageous action in regard to the pressure and flow conditions within the extrusion head. Furthermore, that design configuration means that the extrusion head is comparatively long.
A still further apparatus for use in the production of hollow bodies from thermoplastic material by extrusion blow molding as disclosed in DE-A-36 35 334 comprises an extrusion head for the production of preforms, the wall of which comprises a laminate material. In that arrangement, the extruders for the supply of the materials for making the preforms are directly connected to the piston so that they are required to participate in the movements of the piston, thus involving an additional structural expenditure and also requiring comparatively large masses to be set in motion.